This application is based on application No. H10-158981 filed in Japan, the contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a device and method for starting a brushless motor incorporating a magnetic field sensor for detecting a pole position of a permanent magnet rotor.
2. Description of the Related Art
Brushless motors are used widely in consumer appliances and electronics today as a result of demand for products with longer service life and high reliability. The air conditioner using brushless motors for the compressor motor and the fan motor have been developed and commercialized.
Brushless motors used for fan motors in particular have a magneto-electric conversion device or a magnetic field sensor such as a Hall IC for detecting the pole position of a permanent magnet of the rotor in the motor. The output signal from the magneto-electric conversion device can then be used to detect rotational speed or position of the rotor, or control driving signals for on/off operation of switching devices within an inverter.
A typical method for drive control of a brushless motor having magneto-electric conversion devices according to the related art is described below.
FIG. 15 shows a typical drive device for a brushless motor. The brushless motor 1 has Hall ICs 1d, 1e, and 1f as magneto-electric conversion devices. The Hall ICs 1d, 1e and 1f output respective signals to a controller 6xe2x80x2.
Drive signals are generated based on the output signals from the Hall ICs 1d-1f to control the on/off operation of switching devices 4a-4f in an inverter 4, and are output from the controller 6xe2x80x2. The drive signals control the terminal voltages of the brushless motor 1. Current thus flows to the windings of the stator 1a, 1b and 1c, and the brushless motor 1 turns.
The general starting method of a brushless motor having a magneto-electric conversion device is described next.
For convenience of discussion, it is assumed that the Hall ICs 1d, 1e and 1f output xe2x80x9cHighxe2x80x9d when they are near to the north pole of a permanent magnet composing a rotor 1g, and output xe2x80x9cLowxe2x80x9d when they are near to the south pole. It is also assumed that when the switching device 4a, 4b or 4c on each upper arm of the inverter 4 is on and current flows from the stator winding 1a, 1b or 1c to an neutral point, the side of the winding facing to the magnet rotor is north, and that when the switching device 4d, 4e or 4f on each lower arm is on and current flows from the neutral point to a winding, the side of the winding facing to the rotor 1g is south.
When the rotor 19 is positioned relative to the Hall ICs 1d-1f as shown in FIG. 16, the output signal from Hall IC 1d is Low, and the output signals from Hall ICs 1e and 1f are High. It will thus be obvious that the position of the rotor 1g can be determined based on the output signals from the Hall ICs.
After the position of the rotor 1g is recognized based on the Hall IC output signal such that the rotor 1g is stopped at the position as shown in FIG. 16, the switching device 4a of the upper arm to U-phase and the switching device 4f of the lower arm to W-phase are turned on. As a result, the side of the U-phase winding la facing to the rotor 1g is magnetized to north, and the side of W-phase winding 1c facing to the rotor 1g is magnetized to south. The U-phase winding 1a and the rotor 1g thus attract, and W-phase winding 1c and the rotor 1g repulse. This allows the rotor 1g to begin to turn. The switching devices of the inverter are thereafter switched alternately on and off based on the output signals from the Hall ICs, repeatedly changing the attraction-repulsion relationship between the windings and the magnet rotor. The brushless motor is thus rotationally driven as the magnet rotor chases the alternating magnetic field.
This conventional control method of the brushless motor has the following problem. The output signals from the Hall ICs may be inconstant in the case where some of Hall ICs are near to the boundary between magnetic poles of the rotor 1g, and the rotor 1g tremors due to vibration before start. The inconstant signals are applied to a microprocessor and used to control driving of switching devices. However this inconstant signals make it difficult for the microprocessor to determine which switching devices of the inverter should be on, and may obstruct the normal starting process.
An object of the present invention is directed to a device for starting a brushless motor whereby the brushless motor can be easily and reliably started, and an overcurrent is not produced when the brushless motor is started.
In order to overcome the disadvantages of the prior art, a device is provided for starting a brushless motor having a plurality of stator windings forming predetermined phases, a rotor with a permanent magnet and a plurality of magnetic field sensors for detecting a pole position of the rotor.
The starting device comprises an inverter and a controller. The inverter has a plurality of switching elements in a bridge connection for supplying current to the brushless motor. The controller generates and applies, based on output signals from the magnetic field sensors, control signals for controlling operation of the switching elements of the inverter. Before brushless motor starting the controller positions the rotor by supplying current between at least two phases of the stator windings, and then outputs the control signals based on signals output from the magnetic field sensors to start the brushless motor. As a result, stable output signals can be obtained from the magnetic field sensors regardless of the permanent magnet rotor position before motor starting, and the motor can be reliably started.
The controller may position the rotor by supplying current from one phase to another phase of the stator windings when the level inversion point of signal output from each magnetic field sensor is within an electrical angle range of xc2x115 degrees to the zero cross point of the stator winding induced voltage.
Alternately, the controller may position the rotor by supplying current from one phase to the other two phases of the stator windings when the level inversion point of signal output from each magnetic field sensor is within an electrical angle range from +15 degrees to +45 degrees of the zero cross point of the stator winding induced voltage. This makes it possible to limit the maximum current supply required to position the rotor when the permanent magnet rotor is slightly turning at the controller""s positioning the rotor.
Furthermore, the controller may brake the rotor by supplying current to the switching devices on lower arms of the bridge before positioning the rotor.
Furthermore, the controller may detect rotational speed of the brushless motor based on the output signals from the magnetic field sensors, and brake the rotor until the detected rotational speed is lower than a predetermined level.
This invention also provides a method for starting a brushless motor having a plurality of stator windings, a rotor with a permanent magnet and a plurality of magnetic field sensors for detecting a pole position of the rotor, the method comprising the steps of positioning the rotor by supplying current to stator windings of predetermined phases to set the rotor at a start position where the signals from the magnetic field sensors can be reliably detected, and starting the brushless motor.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.